JP2002189287A - Mask blank evaluating method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mask blank evaluating method and apparatus which are capable of easily evaluating the entire part of mask blanks and are high in throughput. SOLUTION: The pressure to be added to a measuring sample is changed by changing the pressure in a pressure chamber 1. The pressure in the pressure chamber 1 is measured by a pressure gage 7. The silicon film in the measuring sample is irradiated with a laser beam 9 through a translucent film 10 and the reflected light meeting a bulging from the silicon film is imaged by a one- dimensional photodetector 11, such as a photodiode array. A personal computer 4 calculates the bulging of the measuring sample from the spread of the reflected light detected by the one-dimensional photodetector 11. This measurement is carried out plural times by changing the pressure and the personal computer 4 determines the residual stress in the measuring sample and the Young's modulus of the measuring sample from the pressure and the bulging of the measuring sample.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for evaluating a transfer mask blank (a mask in this specification is a concept including a reticle) used in a reduction projection exposure apparatus using an electron beam, an ion beam, an X-ray, or the like. About
More specifically, the present invention relates to an apparatus for measuring the internal stress of a mask blank.

[0002]

2. Description of the Related Art In recent years, with the increase in the degree of integration of integrated circuits, charged particle beams, for example, electron beams, have been replaced by photolithography using light, which has conventionally been the mainstream of means for forming fine patterns. A new exposure method using an ion beam, an ion beam, or X-rays has been studied and put to practical use. The transfer mask used for these exposures is a stencil-type mask in which a transparent portion opening for an electron beam, an ion beam, an X-ray, or the like is formed on a thin film substrate made of silicon in accordance with a predetermined pattern shape. Used.

FIG. 6 is a schematic view of a reticle used as a transfer mask. This reticle is 6inch ~
It is manufactured using a silicon wafer 21 of an arbitrary size of 12 inches. The reticle is reinforced by a support portion 22 having a beam structure, and the size of a window exposed in one exposure is from sub mm to several mm.
It is called a subfield in the order of mm. A desired transfer pattern is formed on the free-standing thin film portion 23 in the window constituting the subfield.

The silicon membrane used for these mask blanks is manufactured by a method as shown in FIG. In this example, an SOI wafer 24 formed by a lamination method using heat melting is used. As shown in FIG. 7A, the SOI wafer 24 includes a supporting silicon substrate 25, a silicon oxide layer 26, and a silicon active layer 27.

As shown in FIG. 7B, a resist 28 is applied as a mask material at the time of dry etching on the supporting silicon substrate 25 side of the SOI wafer 24, and a part thereof is formed into a window pattern shape 2.
Etch 9 In this embodiment, the resist 28 is used as a mask material. However, depending on etching conditions, a silicon oxide film having a thickness of about several μm may be formed and a silicon oxide film may be used as a mask material.

After that, using the resist 28 as a mask, the silicon support substrate 25 is dry-etched under silicon etching conditions. Since the silicon oxide is hardly etched under the silicon etching condition, the silicon oxide layer 2
6 serves as an etching stop layer, and a thin film having a window shape 30 as shown in FIG. 7C is formed.

Thereafter, when the silicon oxide layer 26 is removed with a mixed solution of ammonium fluoride and hydrofluoric acid, the silicon active layer 27 becomes a membrane as shown in FIG. Thereafter, the stencil type transfer mask is completed by patterning the silicon membrane 27.
The silicon active layer 27 may be patterned at the beginning of this step.

However, in the conventional SOI wafer 24 as shown in FIG. 7, the silicon support substrate 25 and the silicon active layer 27 are heated to a temperature of a few hundred degrees and the silicon oxide layer 26 is thermally melted. We are doing vending through. Therefore, due to the difference in the thermal expansion coefficient between the silicon active layer 27 and the silicon oxide layer 26, a thermal residual stress of compression is generated in the silicon active layer 27 when the temperature returns to normal temperature. Therefore, there is a big problem that the thin film is bent when the self-standing thin film is formed. In addition, various techniques have made it possible to control the film stress of the tensile response from the compressive stress, but if the tensile stress is too strong, the deformation of the pattern due to the stress when forming a fine pattern The problem occurs. That is,
A technique for evaluating the internal stress of a membrane is a very important technique for producing a mask blank.

Conventionally, a bulge method has been used as one means for measuring the internal stress and Young's modulus of a self-supporting thin film.
The experimental principle of the bulge method will be briefly described below. In the bulge method, the residual internal stress of the thin film and the Young's modulus can be simultaneously derived from the amount of deformation of the thin film when a static load is applied. The sum of the elastic energies when a load is applied to the thin film is expressed as the sum of the strain energy caused by the load and the strain energy caused by the internal stress, and is stabilized under the condition that the sum of the elastic energies becomes equal to the pressurizing energy. This can be expressed by equation (1).

Here, P is a pressure, σ is an internal stress, t is a film thickness, h is a swelling amount, a is a length of one side of a window, E is a Young's modulus, and C ₁ and C ₂ are windows of a self-supporting thin film. It is a constant determined from the shape and Poisson's ratio. The first term on the right side corresponds to elastic energy due to internal stress, the second term corresponds to elastic energy due to deformation, and the left side corresponds to pressurizing energy. P = C ₁ σth / a ² + C ₂ Eth ³ / a ⁴ … (1) As can be seen from equation (1), by measuring the amount of swelling of the thin film when the applied pressure is changed, the internal stress and It is possible to simultaneously guide the Young's modulus. Qualitatively, the higher the Young's modulus, the smaller the swelling amount, and similarly, the higher the internal stress, the smaller the swelling amount.

FIG. 8 shows a configuration diagram of an experimental device of a conventional internal stress / Young's modulus measuring device. The measurement sample 32 is set on the pressure adjustment tank 31. The contact portion between the measurement sample 32 and the pressure adjustment tank 31 is sealed with an O-ring 33 or the like. The chip (measurement material 32) is supported at four corners by a holding plate 35 fixed with screws 34. The pressure adjusting device 36 adjusts the pressure in the adjustment tank 31 to a desired pressure via a regulator. Further, a pressure gauge 37 is installed in the pressure adjusting tank 31 and constantly monitors the pressure. Pressure regulator 36
Sample 32 (self-supporting thin film) when pressure is changed by
Is measured using a bulging amount measuring means 38 such as a palpation type step difference measuring device, a non-contact interferometer type surface shape measuring device, and a microscope equipped with a micrometer in the Z direction. Then, the internal stress and Young's modulus, which are unknowns, are obtained by fitting the measurement result to the equation (1).

[0012]

However, the conventional swelling amount measuring means as shown in FIG. 8 has various problems. For example, when using a palpation type step difference measuring device, the silicon film itself is deformed by the palpation pressure of the probe, and it is difficult to measure a correct swelling amount at a certain pressure. In a non-contact interference type surface profile measuring instrument,
Since it takes several tens of seconds to measure one point, it takes a very enormous amount of time to measure the entire mask blank. In addition, before starting the measurement, it is necessary to perform an alignment for making the measurement sample horizontal, so that four stages are required. Similarly, a microscope with a micrometer requires a long measurement time and is difficult to automate.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a mask blank evaluation method and apparatus which can easily evaluate the entire mask blank and have high throughput.

[0014]

A first means for solving the above problem is a method for measuring the internal stress of a thin film portion of a mask blank, which comprises applying a pressure to the thin film portion via a fluid, The amount of deformation of the thin film portion at that time is measured by detecting the spread of the reflected light of the light beam applied to the thin film, and the internal stress of the thin film portion of the mask blank is determined using the pressure and the measured deformation amount. It is a mask blank evaluation method (claim 1) characterized by measuring.

The difference between this means and the conventional example is that a method of detecting the spread of reflected light of a light beam applied to the thin film is used for measuring the amount of deformation (bulge) of the thin film portion. . As will be described later in the embodiments of the invention,
A predetermined relational expression is established between the amount of deformation of the thin film when pressure is applied to the thin film portion via a fluid and the spread of the reflected light. Since the optical lever principle is used for the measurement, a small deformation amount can be measured accurately and quickly. Therefore, it is possible to provide a mask blank evaluation method that has higher accuracy and shorter measurement time than the conventional measurement method. In addition, since the basic measurement principle is the same as the conventional method, it goes without saying that the Young's modulus can be measured together with the internal stress.

A second means for solving the above-mentioned problem is as follows.
An apparatus for measuring the internal stress of a thin film portion of a mask blank, comprising: a pressure applying device for applying a plurality of predetermined pressures to a thin film portion to be measured via a fluid; and a pressure measuring device for measuring the applied pressure. A light beam irradiating device for irradiating a thin film to be measured with a light beam; a reflected light distribution measuring device for measuring a spread of a light beam reflected by the measured thin film; and a deformation amount of the measured thin film from the spread of the light beam. A mask comprising: a deformation amount calculating unit; and an internal stress calculating unit that calculates an internal stress of the thin film to be measured from the plurality of measured pressures and the deformation amount of the thin film at that time. This is a blank evaluation apparatus (Claim 2).

In this means, the pressure applying device
A predetermined pressure is applied to the thin film portion to be measured, and the pressure is measured by a pressure measuring device. The expression "a plurality of pressures" means that different pressures are applied in several steps and a measurement is performed each time. On the other hand, the amount of deformation of the thin film deformed by applying pressure is measured by a reflected light distribution measuring device, and the spread of the light beam emitted from the light beam irradiation device and reflected by the thin film is measured. It is calculated by: Then, the internal stress calculating means calculates the measured internal stress of the thin film from the plurality of measured pressures and the deformation amount of the thin film at that time. As described above, the internal stress and the Young's modulus can be simultaneously obtained by fitting calculation or the like by switching the pressure in several stages and measuring the deformation amount each time.

In the present means, as in the case of the first means, the mask blank can be evaluated with high accuracy and in a short measuring time as compared with the conventional measuring apparatus.
In order to evaluate the entire self-supporting thin film portion constituting the subfield in the mask blank, for example, the optical system is fixed, and the mask blank and the pressure applying / measuring system are X-
Mount it on a two-axis moving device in the Y direction and move it, or fix the mask blanks and pressure application / measurement system, and change the optical system to XY
What is necessary is just to mount it on a two-axis moving device and move it.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic block diagram of a mask blank evaluation apparatus which is an example of an embodiment of the present invention. In FIG. 1, 1 is a pressure vessel, 2 is an XY stage,
3 is an XY stage controller, 4 is a personal computer, 5 is a low voltage generating power supply, 6 is a regulator, 7 is a pressure gauge, 8 is a voltmeter, 9 is a laser beam, 10 is a semi-transmissive mirror, and 11 is a one-dimensional beam. The detector 12 is a one-dimensional photodetector controller.

The measurement sample is set on the pressure tank 1, and the pressure tank 1 is set on the XY stage 2. The XY stage 2 can be controlled by a personal computer 4 via an XY stage controller 3. The pressure in the pressure vessel 1 is controlled by a personal computer 4
Thus, the voltage generated by the low-voltage power supply 5 is controlled and the voltage applied to the electric regulator 6 is controlled. By changing the pressure in the pressure vessel 1, the pressure applied to the measurement sample is changed. The pressure in the pressure vessel 1 is measured by a pressure gauge 7, and the measurement result is converted into a voltage and input to the personal computer 4 via a voltmeter 8.

Laser light 9 is applied to the silicon film of the measurement sample.
Is irradiated through the semi-transmissive mirror 10, and reflected light from the silicon film according to the bulge is reflected on a photodiode array or the like.
An image is taken by the two-dimensional photodetector 11. In this embodiment, 1
Although the two-dimensional photodetector 11 is used, a two-dimensional photodetector may be used. The reflected light from the silicon film detected by the one-dimensional photodetector 11 is converted into data by the one-dimensional photodetector controller 12 and sent to the personal computer 4. In this embodiment, as described above, one personal computer 4 controls the entire system,
Data income, calculations are implemented.

Next, the flow of data processing in the mask blank evaluation apparatus shown in FIG. 1 will be described. The personal computer 4 controls the voltage applied to the regulator 6 to apply a desired initial pressure to the pressure vessel 1. The pressure at that time is measured by a pressure gauge 7 and input to the personal computer 4 via a voltmeter 8 and recorded. The laser light 9 is constantly irradiated on the measurement sample, and the one-dimensional detector 11 detects reflected light from the silicon film according to the swelling amount.

The intensity distribution data of the reflected light from the measurement sample (silicon film) detected by the one-dimensional photodetector 11 is also input to the personal computer 4 via the one-dimensional photodetector controller 12. Personal computer 4
From the distribution width of the reflected light detected by the one-dimensional detector 11,
The swelling amount of the silicon film is calculated and recorded in a file in which the pressure is recorded. Through this series of steps, the amount of window bulge at the desired initial stress is recorded.

Thereafter, by changing the applied voltage to the regulator 6 and recording the amount of swelling of the silicon film when several points of pressure are changed, the data acquisition of one reticle subfield is completed.

When the data acquisition for one subfield is completed, the XY stage 2 is controlled by the personal computer 4 to move the stage to a desired subfield in the mask blank. In the next subfield, the same control and measurement as described above are performed, and this is repeated to collect data for the subfield of the entire mask blank.

The personal computer 4 derives the film quality (residual internal stress / Young's modulus) for each subfield of the entire mask blank based on these measurement data. This calculation is performed by fitting each measurement data to the above equation (1) using the least squares method or the like.

FIG. 2 shows an example of a series of measurement results in one subfield. The horizontal axis is the applied pressure, and the vertical axis is the swelling amount of the window constituting the subfield at that time. It can be seen that the amount of window swelling increases with increasing pressure. The solid line shows the result of the fitting. From the figure, it can be seen that the fitting calculation is performed with high accuracy.

Next, in the embodiment shown in FIG. 1, the principle of irradiating the silicon film with a light beam and deriving the film swelling amount from the reflected light will be described with reference to FIG. In FIG.
13 is a silicon film inflated by pressure, 14 is a silicon film substrate, 15 is laser irradiation light, 16 is an intersection between the outermost part of the incident laser light and the silicon film, 1
Reference numeral 7 denotes a center of curvature radius, reference numeral 18 denotes reflected light, and reference numeral 19 denotes a projection surface.

FIG. 3 shows only a half of the subfield having a side length of 2C from the center line. It is assumed that the silicon film 13 is deformed into a curved surface having a radius of curvature R by the applied pressure. When a laser irradiation light 15 having a radius b is incident from a direction perpendicular to the silicon film substrate 14,
The silicon film 13 acts as a spherical mirror, and the laser light is diverged as shown in the figure.

A straight line connecting an intersection 16 between the outermost portion of the laser beam incident on the silicon film 13 and the silicon film and a center of curvature radius 17 having a curvature similar to the deformation of the silicon film;
Assuming that the angle between the laser light and the incident optical path is θ, the incident laser light 15 is reflected light 18 at an angle of 2θ from the incident optical path.
It is diverged as Assuming that half of the spread width of the reflected light on the projection surface 19 (in this embodiment, a one-dimensional photodetector) is d, and that the distance from the silicon substrate 14 to the projection surface 19 of the diverging laser light is L, the silicon at the center is The swelling amount h of the film is
Approximately expressed as equation (2).

[0031]

(Equation 1)

FIGS. 4 (a) and 4 (b) are schematic diagrams (a) showing how the silicon film expands when pressure is applied, and schematic diagrams (b) of the reflected laser light projected on the screen. . The bulge of the silicon film is represented by a contour line. In the figure, the inclination of the AA "cross section is steeper than the inclination of the BB" cross section, so that the divergence angle on the silicon film becomes larger and the width of the reflected light projected on the screen becomes wider. Therefore, a star-shaped projection image as shown in FIG. The divergence width of the beam on the projection plane, which is used in the principle of deriving the amount of window bulging in FIG.

FIG. 5 shows an example of the result of estimating and calculating the measurement accuracy required for measuring the spread of the reflected light. This shows a calculation result of a laser divergence angle when a laser beam having a diameter of 1 mm is incident on a 1 mm square silicon thin film. The horizontal axis indicates the bulging amount of the central portion of the silicon window, and the vertical axis indicates the spread angle of the laser reflected light.
From the graph, the beam divergence angle is about 80 mrad when the swelling amount is 10 μm. This is a spread width of about 23 mm when the distance to the projection plane (the one-dimensional photodetector light receiving plane) is 138 mm. For example, when a one-dimensional photodetector of 1024 channels (each channel has a width of 25 μm) is used, the measurement accuracy of the swelling amount per channel is 0.1 μm.
, Which is about ± 1 MPa in terms of a stress value. Therefore, it can be seen that a very simple mechanism can achieve a high bulging measurement accuracy. It goes without saying that the longer the distance to the projection plane, the higher the accuracy of measuring the spread width.

[0034]

As described above, in the present invention, a simple device utilizing an optical lever is used to measure the amount of swelling of the free-standing thin film, so that the swelling of the free-standing thin film can be accurately and quickly performed. The amount can be measured. Therefore, evaluation of film quality of mask blanks (residual internal stress and Young's modulus)
Can be performed simply and with good throughput.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of a mask blank evaluation apparatus which is an example of an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a relationship between an applied pressure and a bulging amount in one subfield.

FIG. 3 is a diagram for explaining a principle of irradiating a silicon film with a light beam and deriving a film swelling amount from reflected light.

FIG. 4 shows how the silicon film swells when pressure is applied,
FIG. 3 is a schematic diagram showing projection lines of reflected laser light projected on a screen.

FIG. 5 is a diagram illustrating an example of a result obtained by estimating and calculating measurement accuracy required for measuring the spread of reflected light.

FIG. 6 is a schematic view showing an example of a reticle used as a transfer mask.

FIG. 7 is a diagram illustrating an example of a method for producing a mask blank.

FIG. 8 is a diagram showing a configuration of an experimental device of a conventional internal stress / Young's modulus measuring device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Pressure tank, 2 ... XY stage, 3 ... XY stage controller, 4 ... Personal computer, 5 ... Low voltage generation power supply, 6 ... Regulator, 7 ... Pressure gauge, 8 ...
Voltmeter, 9 laser light, 10 semi-transmissive mirror, 11 one-dimensional photodetector, 12 one-dimensional photodetector controller, 1
Reference numeral 3 denotes a silicon film, 14 denotes a silicon film substrate, 15 denotes a laser irradiation light, 16 denotes an intersection between the outermost portion of the incident laser light and the silicon film, 17 denotes a radius of curvature, 18 denotes reflected light, and 19 denotes a projection surface.

Claims (2)

[Claims] 1. A method for measuring the internal stress of a thin film portion of a mask blank, comprising applying a pressure to the thin film portion via a fluid, and measuring a deformation amount of the thin film portion at that time by a reflection of a light beam applied to the thin film. A method for evaluating a mask blank, characterized in that the measurement is performed by detecting the spread of light, and the internal stress of the thin film portion of the mask blank is measured using the pressure and the measured amount of deformation. 2. An apparatus for measuring the internal stress of a thin film portion of a mask blank, comprising: a pressure applying device for applying a plurality of predetermined pressures to a thin film portion to be measured via a fluid; A pressure measuring device for measuring a light beam irradiating device for irradiating a light beam to the thin film to be measured; a reflected light distribution measuring device for measuring the spread of the light beam reflected by the thin film to be measured; Deformation amount calculation means for calculating the deformation amount, and internal stress calculation means for calculating the internal stress of the thin film to be measured from the plurality of measured pressures and the deformation amount of the thin film at that time. The mask blank evaluation apparatus characterized by the above-mentioned.